Ink compositions containing colorants used to generate images onto an image receiver can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists in solution as a fully solvated dye species that consists of one or more dye molecules. The use of pigments to form images onto a receiver presents a number of significant challenges. Pigment particles must be adequately dispersed in a suitable carrier, can be colloidally destabilized by ink components and can present settling problems during storage of the ink. Pigments can be difficult to jet through the small orifices of an inject printer and can clog the nozzles of the printhead. Pigments usually reside at the surface of the image receiver to which they are printed and image durability can be inadequate. However, pigments are desirable colorants since they are typically far more resistant to light and ozone induced fading than dyes. This provides an additional advantage for pigment-based images in that they can be printed to microporous image receivers without much concern for their inherent stability. Microporous receivers are highly desirable since the printed ink dries rapidly thereby allowing a customer to handle prints shortly after printing.
Dye-based images are typically advantaged over pigment-based images with respect to achieving high-density images. However, dye-based images are particularly susceptible (and disadvantaged to pigment-based images) to ozone induced fade mechanisms when printed onto a microporous image receiver. Microporous receivers are typically designed with inorganic particles that create small voids that are capable of absorbing ink by capillary forces very quickly. Dyes generally penetrate into the pores of the microporous receiver and generally very little of the dye resides on the surface of the receiver. However, these same pores in the receiver allow a relatively open structure for compounds such as ozone to attack the dye-based image. Furthermore, the inorganic particles contained in the image receiver can themselves liberate free radicals that accelerate the fading of the dye-based image.
Various approaches are known in the art to improve the stability of dye-based images with respect to ozone induced fade mechanisms. In one approach, stabilizer compounds (such as antioxidants or hindered amine light stabilizers) are added to the image receiver in order act as sacrificial scavengers of free radicals and ozone. Although this approach can improve dye fade, the sacrificial stabilizer compounds can cause color changes in the image receiver. Furthermore, this approach typically requires a large amount of stabilizer to be contained in the image receiver and ozone can still react with the dyes. In a second approach, the inorganic particles that comprise the image receiver can be modified with stabilizing groups through reactive chemistry. Again this requires substantial modification of the entire image receiver and has a negative impact on the cost of the receiver.
Yet another approach to improving the fade performance of dye-based images is to print the dye-based inks onto swellable image receivers. However, swellable image receivers are significantly disadvantaged to microporous image receivers with respect to dry time.
It is also known in the art to apply colorless or clear inks as overcoats to dye-based images for the purpose of improving image durability. Typically, these clear inks contain film-forming polymers, which are in the form of water-soluble polymers or water-dispersible emulsion particles. Although the polymers used in clear inks can improve the durability of images on receivers they are generally ineffective at improving the fade resistance of dye-based images.
Inkjet printing is one application where dye-based are applied to image receivers with the expectation of generating high quality images. Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal ink jet (TIJ)) and piezoelectric transducers. In another process known as continuous ink jet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump.
Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling. However, in many of these applications the stability of the images can be significantly degraded by ozone. This is especially true for photographic quality images, which are expected to last for several years without substantially fading or significantly changing in color rendition. Therefore, it would be desirable to provide dye-based images that are capable of withstanding the effects of ozone induced fade, and over a wide variety of media.
U.S. Pat. No. 6,854,840 suggests the use of smectic liquid crystal materials in gel form to help improve blocking resistance, durability, gloss, optical density and to protect an image from ozone degradation. Inkjet inks are free-flowing liquid dispersions or solutions capable of ejection by a printer nozzle. If the material gels or becomes too viscous, printing can be impaired, especially when using a thermal printhead. Although application of the materials is suggested to be possible by ink jet methods, it is impractical to apply gelled materials via inkjet printheads, particularly thermal printheads, due to the high viscosity of such materials and the corresponding tendency of such materials to fail to achieve the desired ejection velocity and the consequent tendency to plug the nozzles of the ink jet printer. Further, it is desirable that an ink be usable with any kind of printhead.
Thus, it is a problem to be solved to provide a jetable overcoat clear ink containing inorganic particles that inhibit ozone degradation of a resulting image without occasioning jetting difficulties and without employing specialized ink receiving media.